Isochrony and rhythmic interaction in ape duetting

Abstract

How did rhythm originate in humans, and other species? One cross-cultural universal, frequently found in human music, is isochrony: when note onsets repeat regularly like the ticking of a clock. Another universal consists in synchrony (e.g. when individuals coordinate their notes so that they are sung at the same time). An approach to biomusicology focuses on similarities and differences across species, trying to build phylogenies of musical traits. Here we test for the presence of, and a link between, isochrony and synchrony in a non-human animal. We focus on the songs of one of the few singing primates, the lar gibbon ( Hylobates lar ), extracting temporal features from their solo songs and duets. We show that another ape exhibits one rhythmic feature at the core of human musicality: isochrony. We show that an enhanced call rate overall boosts isochrony, suggesting that respiratory physiological constraints play a role in determining the song's rhythmic structure. However, call rate alone cannot explain the flexible isochrony we witness. Isochrony is plastic and modulated depending on the context of emission: gibbons are more isochronous when duetting than singing solo. We present evidence for rhythmic interaction: we find statistical causality between one individual's note onsets and the co-singer's onsets, and a higher than chance degree of synchrony in the duets. Finally, we find a sex-specific trade-off between individual isochrony and synchrony. Gibbon's plasticity for isochrony and rhythmic overlap may suggest a potential shared selective pressure for interactive vocal displays in singing primates. This pressure may have convergently shaped human and gibbon musicality while acting on a common neural primate substrate. Beyond humans, singing primates are promising models to understand how music and, specifically, a sense of rhythm originated in the primate phylogeny.

Recursive self-embedded vocal motifs in wild orangutans

“…Given their conspicuousness, loud calls represent one of the most studied aspects of primate vocal behaviour (Wich and Nunn, 2002), but their rhythmic patterns have only recently started to been characterized with precision (Clink et al, 2020;De Gregorio et al, 2021;Gamba et al, 2016). Besides our analyses, there are remarkably few confirmed cases of vocal isochrony in great apes (but see Raimondi et al, 2023), but the behaviours that have been rhythmically measured with accuracy have been implicated in the evolution of percussion (Fuhrmann et al, 2015) and musical expression (Dufour et al, 2015;Hattori and Tomonaga, 2020), such as social entrainment in chimpanzees in connection with the origin of dance [a capacity once also assumed to be neurologically impossible in great apes (Fitch, 2017;Patel, 2014)]. This opens the intriguing, tentative possibility that recursive vocal combinatorics were first and foremost a feature of proto-musical expression in human ancestors, later recruited and "re-engineered" for the generation of linguistic combinatorics.…”

Isochrony in barks of Cape fur seal (Arctocephalus pusillus pusillus) pups and adults

“…We do not know whether for a Cape fur seal maintaining isochrony is inherently important (e.g., conveys socially important information such as suggested for Nomascus and Hylobates gibbons; Raimondi et al., 2023; de Gregorio et al., 2023; Ma et al., 2024), or an artefact of the most efficient way of performing this vocalisation (similar to, e.g., walking), which may require practice to develop. If maintaining a roughly isochronous pattern requires practice, a young animal may put more effort and slightly overaccentuate the rhythmic action before it becomes fluent in it.…”

Manual preference, performance, and dexterity for bimanual grass‐feeding behavior in wild geladas (Theropithecus gelada)

“…Data on the onsets of successive hand movements were collected as inter‐onset intervals t k (De Gregorio et al, 2023; Raimondi et al, 2023) and were not normally distributed. Considering the two hands separately, we calculated t k s , (i.e., the time between the onset of a hand‐plucking movement) and then calculated the rhythm r k using the formula in Roeske et al (2020).…”

“…Considering the two hands separately, we calculated t k s , (i.e., the time between the onset of a hand‐plucking movement) and then calculated the rhythm r k using the formula in Roeske et al (2020). To evaluate the occurrence of isochrony (following De Gregorio et al, 2021; Raimondi et al, 2023), we centered the on‐isochrony ratio range around 1:1 and divided the ratio distribution into on‐isochrony and off‐isochrony (see De Gregorio et al, 2023 for details). After counting all ratios that fell in each on‐ and off‐isochrony ratio range for each session of each individual, we test whether the counts into the on‐isochrony ratio range differed from the off‐isochrony ratio range using a paired‐data Wilcoxon test in R (R Core Team, 2023).…”